Multiple Approach to Analysis of H2O Injection Into a Gas Turbine

Open Access Journal Journal of Power Technologies 96 (3) (2016) 200–205

journal homepage:papers.itc.pw.edu.pl

Multiple approach to analysis of H2O injection into a gas turbine

Paweł Ziółkowskia,∗, Janusz Badura, Krzysztof Jesionekb, Andrzej Chrzczonowskib

aEnergy Conversion Department, The Szewalski Institute of Fluid-Flow Machinery Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk,´ Poland; bWrocław University of Technology, Wybrze˙zeWyspianskiego´ 27, 50-370 Wrocław, Poland

Abstract

This paper presents a thermodynamic analysis of the Brayton cycle and an upgrade to it involving the injection of H2O into the gas turbine cycle. Upgrades are generally considered to be environmentally-friendly solutions and lead to an increase in efﬁciency, but in the literature there is no clear answer as to what type of upgrade is the best. Computational Flow Mechanics codes have been used for numerical analysis of: the Brayton simple cycle, the Brayton cycle with water injection into the compressor and with regeneration prior to the combustion chamber, the STIG (steam injection gas turbine), and the CSTIG (combined steam injection gas turbine) system. Different ways of analyzing H2O injection into the gas turbine cycle are discussed. Keywords: gas turbine; steam injection; regeneration; water injection; water recovery

1. Introduction retrofitted gas turbines have been widely analyzed and developed [4, 9]. Cheng suggested that the whole steam pro- Today, almost all developed countries, including Poland, are duced in the heat recovery steam generator (HRSG) can be faced with the problem of an insufficient amount of electric- used for injection into a gas turbine combustion chamber, ity. An additional difficulty is the need to meet environmental which results in an increase in both power and efficiency as standards set by EU Directive No. 2010/75/EU [1]. Ecolog- well as a reduction in NOx [9]. ical solutions delivering growth in electric capacity include However, CSTIG and STIG are gas-steam systems, in upgrading existing gas unit and units of gas-steam power which a Brayton gas cycle is combined with a steam cycle in plants in systems with injection of water or steam to the gas the combustion chamber. In a gas turbine, both the exhaust turbine [2–5]. The power and efficiency of the basic Brayton and the steam generated in the heat recovery steam genera- cycle can be increased by using STIG (steam injection gas tor expand and, consequently, the power of the Brayton cycle turbine), CSTIG (combined steam injection gas turbine), the increases [9–11]. The specification of these systems makes injection of water into the compressor and regeneration prior high electrical efficiency achievable in small systems as well. to the combustion chamber and combined cycle. STIG and In addition, the exhaust gases leaving the combustion cham- CSTIG are particularly beneficial, while the demand for heat ber contain only small amounts of toxic components, so ex- and power plants is falling. In turn, STIG and systems of pensive exhaust treatment systems are not required [10, 12]. water injection into the compressor and regeneration before Particular advantages are achieved together with a reduction the combustion chamber have the potential to increase the in emissions of nitrogen oxides in gas turbines that gener- power and efficiency of the unit during repair of the steam ate predominantly through the Zeldowicz mechanism (called turbine. Moreover, the STIG and CSTIG systems provide thermal mechanism) [13]. As a result of steam injection into flexible operation of power plants with optimal utilization of a combustion chamber, the temperature of the combustion generated steam [6–8]. Many studies have been carried process decreases and consequently emissions of nitrogen out on gas turbines with the steam injection. Ever since oxides are reduced [4, 14]. It should be added that in some Cheng proposed a gas turbine with steam injection in 1978, applications gas turbines are used as a heat supply for the stripping process in a supercritical CHP plant integrated with a carbon capture installation. This helps cut carbon emis- ∗Corresponding author sions [15]. [email protected] Email addresses: (Paweł Ziółkowski), The first proposed upgrade to the Brayton cycle with steam [email protected] (Janusz Badur), [email protected] (Krzysztof Jesionek), [email protected] (Andrzej injection system, as presented by Saada and Cheng, con- Chrzczonowski) cerned the General Electric LM2500 turbine [4]. At present, Journal of Power Technologies 96 (3) (2016) 200–205 steam injection into the combustion chamber is used in turbines by leading companies such as General Electric, Rolls- Royce, Kawasaki [4, 16]. In the paper [17] it is shown that applying STIG to a basic gas turbine cycle of General Elec- tric Frame 6B could increase efficiency from 30% to 40% and power from 38 to 50 MWe. More complex systems ex- ist, e.g. using water injection to compressed air as inter- or inlet-stage cooling [18–20], in humid air (Humidified Air Turbine, Evaporating Gas Turbine, Cascade Humidified Air Turbine) [19, 20], systems connecting the steam injection with regeneration (RSTIG) [20], systems combining both water and steam injection DRIASI (the dual-recuperated, inter- cooled-after-cooled steam injected cycle) [3] and LOTHECO cycle (low temperature heat combined cycle), using an exter- Figure 1: Schematic diagram of gas-steam cycle with a possi- nal renewable heat source for heating the water condensed bility of transition to the STIG, (GT—gas turbine, C—compressor, CC—combustion chamber, G—generator, HRSG—heat recovery steam from the exhaust gases [3, 12]. generator, ST—steam turbine, P—pump, CON—condenser, WH—heat ex- Another method that would increase the efficiency and changer) [6] power of a gas turbine is the process of spraying water between compressor stages. During spraying, the water evap- orates in contact with the hot and compressed air mass flow CFM (Computational Flow Mechanics) [5, 14, 21, 22] and rate , which in turn lowers the air temperature and reduces a commercial package. Then an analysis of efficiency for the amount of work required to drive the compressor. This three modifications is conducted: in the Brayton cycle with system with inter-stage cooling of air can be successfully water injection to the compressor and the regeneration prior used in high-pressure turbines [20]. to the combustion chamber, and in the STIG and CSTIG Moreover, the construction of a Humidified Air Turbine systems. Additionally, different aspects of H2O injection into (HAT) is based on the concept of combining the regenerative the gas turbine cycle are presented. An economic analysis air preheating for combustion and the injection of water mass was carried out in respect of two units, both approximately flow rate between the compressor and heat exchanger (with- 66 MWe – for a gas-steam unit and a unit in the Cheng sys- out changing the flow of compressed air). Injection of water tem. into the air, before the regenerative heat exchanger, lowers the temperature of the compressed medium and therefore increases the efficiency of heat regeneration. Because of 2. Theory the use of the regenerative heat exchanger in HAT, the opti- mum compression ratios are lower than in the STIG, thereby CFM type numerical codes were used in the analysis of resulting in a higher temperature after the turbine in the thermodynamic cycles. CFM mathematical models (for ex- order of 800◦C. In HAT systems, the temperature distribu- ample in the programs COM-GAS and Aspen Plus) use the tion mediums in the regenerative heat exchanger are signif- balance equations of mass, momentum and energy in the icantly better adjusted than in the heat recovery steam gen- integral form (0D, integrated) [23–25]. In previous works of erator (HRSG) systems with steam injection or gas-steam the authors [6, 7, 24], calculation procedures were presented cycles, therefore an increase in efficiency, even above 0.5, for all components of the turbine set, such as compressors, is observed. But the disadvantage of these systems is that combustion chambers, turbines, pumps and heat exchang- they require changes in construction and, consequently, in- ers. The power, efficiency, steam injection rate and other creased investment outlays [3, 14, 20]. relevant values have been defined in previous papers of the Next, the LOTHECO cycle (LOw Temperature HEat COm- authors [6, 7, 24]. bined cycle) employs an external renewable heat source to Figs 1–3 show schematic diagrams of cycles compared heat up and evaporate the water condensed from the ex- in this work. Firstly, Fig. 1 presents the layout of a gas haust gases [3]. For a temperature range of evaporation and steam power plant with the ability to redirect the steam from below 100◦C to 170◦C, low-quality heat sources are fa- to the combustion chamber (CC), thus using the STIG so- vorably integrated, which under other circumstances cannot lution. Secondly, Fig. 2 shows an arrangement of CSTIG, be utilized for electric power generation, such as: geother- which is a kind of equivalent to STIG, with the exception that mal, solar, etc. Such arrangements are beneficial in terms of the steam generated in the heat recovery steam generator enhanced fuel-to-electricity efficiency compared to the effi- (HRSG) reworks a part of the enthalpy in the steam turbine ciency of an equivalent conventional combined cycle [3, 12]. (ST) before it reaches the combustion chamber (CC) . Fi- Efficiencies above 60% were recently reported [3]. nally, the last of the analyzed systems using water injection The purpose of this article is to examine thermodynamic (W) in the double section compressor (low-pressure C1 and and operating parameters of the Brayton basic cycle and gas high-pressure C2) and with regeneration (R) before the com- and steam cycle by using the available in-house numerical bustion chamber, is shown in Fig. 3.

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there has been substantial progress in the design of condensing heat exchangers, which can be adapted for the purpose of these systems. Placing a condenser (condensing heat exchanger) in the cycle means all the injected steam can be recovered. Various types of heat exchangers adapted to systems with steam or water injection have been studied in the literature [26]. The ones most commonly known are:

• two types of ﬁnned tube exchangers: for exhaust cooled by water and for exhaust cooled by air;

• and direct contact condenser. Figure 2: Schematic diagram of CSTIG system (GT—gas turbine, C—compressor, CC—combustion chamber, G—generator, HRSG—heat The finned tube exchangers are used with mediums with recovery steam generator, ST—steam turbine, P—pump) a large amount of exhaust and a small amount of steam. The cooling of exhaust and condensation of water occurs inside the shell on the tubes’ surface [26]. Projects of water recovery installations for turbine classes M1A-13CC (2.3 MWe); 501KH (6.8 MWe); LM1600 (17 MWe); LM2500 (26.4 MWe); LM5000 (50.7 MWe) have already been tested in STIG systems. It was shown that the water-cooled condenser with finned tubes is the smallest. The advantage of these exchangers (installation) is the lack of moving parts. But in the literature there is also a rotary condenser-separator, which is compact and inexpensive to produce [27]. Usually, after leaving the gas turbine and putting the thermal energy in the heat recovery steam generator, the gas - steam mixture goes to an annular inlet channel with lattice guide vanes. Next the mixture is directed into a vortex chamber and moves along its circumference toward the center, Figure 3: Schematic diagram of the system with regeneration and interstage cooling in the compressor through water injection (C1, C2—low- and-high where there is a pipeline discharging the exhausts, which pressure compressor, R—regenerative heat exchanger, CC—combustion are separated from the steam. Optimal parameters for the chamber, GT—gas turbine, G—generator, W—water injection, P—pump) operation of the device and its detailed characteristics can be found in [27]. Nowadays, shell and tube heat exchangers or finned tube These systems all have their own advantages and disad- exchangers are mostly used in the case of water conden- vantages, which will be briefly described. The STIG system sation from exhaust, especially when it happens in the con- has a simple design and the possibility of easy redirection of densing heat exchanger, in which the temperature of the ex- steam to the gas turbine system. Unfortunately, it does not haust gas flowing out of the boiler decreases, which leads use most of the energy flowing from the steam produced in to condensation of the water and utilization of the latent the heat recovery steam generator (HRSG). An improvement heat [28]. of the present solution is reached in the combined system of CSTIG, but in this situation there is a problem with designing a new steam turbine adapted to the required pressure drop. 3. Results In turn, the water injection solution does not require a heat recovery steam generator (HRSG), but the amount of water As previously mentioned, Fig. 1 shows the gas-steam unit introduced and the pressure at which the water is injected with the possibility of using the steam in the STIG system. into the compressor must be optimized. An additional advan- Part of the mass flow of steam generated in the heat recov- tage of this solution is that it reduces the temperature behind ery steam generator (HRSG) is redirected to the combustion the compressor, thus reducing the compression work and in- chamber (CC), where it is used to reduce the emissions of creasing the degree of regeneration. All these factors make nitrogen oxides (Fig. 4) and to increase the power and ef- it difficult to identify the better solution in thermodynamic and ficiency of the Brayton cycle in the gas turbine (GT). Thus, ecological terms, without prior analysis and comparison of we reduce the steam directed to an extraction-backpressure the results of all three systems. steam turbine, and the heat stream supplied to the recipient decreases. 2.1. Water recovery Fig. 4 shows the relationships between emission of CO2, The disadvantage of the solutions discussed above is the NOx and temperature before the gas turbine tTIT depending need for water recovery from flue gas mixtures. However, on k. In the present case there was a decrease in carbon

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Figure 4: Relationships between emissions of carbon dioxide CO2 and ni- Figure 6: The correlation between the electrical efﬁciency ηel of CSTIG (k = trogen oxides NOx and temperature before the gas turbine tTIT depending 0.04) and a simple gas turbine SGT (k = 0.0) and the compressor pressure ◦ ◦ on k, with m˙ f = const ratio πc for the turbine inlet temperature tTIT = 900 C and 1200 C

Figure 7: System electrical efficiency ηel versus the compression ratio of the Figure 5: Electrical efficiency ηel and the efficiency of the entire power plant compressor’s low–pressure part and relative air humidity ϕ after humidifica- ηEC depending on k, with m˙ f = const tion, with m˙ f = const

dioxide emissions by ∆CO2 = 10 kg/MWh and nitrogen ox- tem, the efficiency increases to ηel = 0.342 and ηel = 0.428 ◦ ◦ ides emissions by ∆NOx = 0.008 kg/MWh. The tendency if respectively tTIT = 900 C and tTIT = 1200 C. The high- of NOx emissions is related to the temperature in the com- est efficiency is achieved with the pressure ratio at a level of bustion chamber and is linear, instead of exponential [5], but πc = 22. In the case of CSTIG, a reduction in emissions is when lowering the temperature, such arrangements may be achieved slightly better than with the STIG system. adopted. Some results of the computations for the total compres- In turn, when analyzing Fig. 5 it should be noted that the sion πc = 15.0, temperature of the medium at the entry ◦ electrical efficiency of the gas-steam cycle in the power plant to the turbine tTIT = 1200 C and the regeneration degree is ηel = 0.4121 [-] and after their transformation in the STIG r = 0.7 are illustrated. The system’s electrical efficiency system, the efficiency increases to ηel = 0.4205 [-]. The high- ηel versus the compression ratio of the compressor’s low- est efficiency is achieved at a ratio of steam injection at the pressure part πLP and relative humidity of the humidified air level of k = 0.13 (ηel = 0.4205 [-]) , where the power of the ϕ22 are shown in Fig. 7. The effect of humidification cooling steam turbine is equal to zero, and the steam produced in the of the compressed air is clearly visible, but it is not identical heat recovery steam generator (HRSG) is injected into the for efficiency and internal unit power. The largest increase combustion chamber (CC). More information about this cy- in efficiency—about 0.045—occurs for the compression ra- cle, and the results of thermodynamic analysis is presented tio πLP = 5. The humidification of air at higher compression in works [5, 6]. ratios πLP results in smaller efficiency increments. In order, Fig. 6 presents the dependence of electrical ef- As Fig. 7 shows, the lower the relative humidity, the weaker ficiency ηel of CSTIG and a simple gas turbine (SGT) from the effect of humidification. Considering that the rate of evap- the compressor pressure ratio πc. The efficiency of a simple oration of water from the surface of droplets decreases as Brayton cycle equals a maximum ηel = 0.304 and ηel = 0.366 relative humidity increases, in order to keep the size of the ◦ ◦ if respectively tTIT = 900 C and tTIT = 1200 C. Then, after humidification chamber small, humidification is terminated the transformation of the Brayton cycle into the CSTIG sys- once suitably lower values of relative humidity ϕ22 are ob-

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